Because of the variable nature of electrical loads, it is desirable to be able to control selectively the loads to which power is supplied. Thus much use is made of time clocks for switching in various loads, for example pumping facilities, hot water storage devices and the like, but time clocks are not in themselves sufficiently versatile to meet extraordinary conditions and are expensive. This problem has been recognised previously, and attempts to improve control over load shedding facilities and other functions have included the provision of the so-called ripple control.
Reference is made to the Proceedings of the Institute of Electrical and Electronic Engineers of America, Vol. 67, No. 2, February, 1979 on page 241, wherein there is an article entitled "Electric Power Load Management: Some Technical, Economic, Regulatory and Social Issues". In the article there is a general review of load shedding techniques utilising ripple control.
In a conventional ripple control system, an audio-frequency signal is produced by a separate generator and then coupled by means of a high voltage capacitor or transformer onto a high voltage bus. The capital plant required is costly, and is physically bulky. Furthermore ripple control is capable of causing inconvenience to consumers under certain conditions because of the relatively large signal magnitude required.
Ripple control receivers as presently used can respond to a relatively wide band width of the ripple frequency, and a range of plus or minus 5% is not uncommon, but such a wide frequency band width of response does not give the security against operation by spurious signals which is sometimes required, and reduces the number of channels which can be used in a relatively small range of signal frequencies. It is found that the frequency at the transmitter of a ripple control unit can vary slightly with both time and temperature and this is also inconvenient and further contributes to a lower standard of security against spurious signals actuating a receive relay than the standard which can be achieved by use of this invention.
An alternative proposal which has been devised by the inventor herein is a voltage distortion caused by switching in a heavy inductive load. This has been proved to be satisfactory but the equipment was found to be expensive and switched such heavy currents that the apparatus operated near its maximum limit.
Further to this, it is known that a system exists whereby the power system voltage wave-form is distorted on the zero crossing, but this again means a very large energy pulse, and each distortion must be separately recognised by the receive circuit.
The main object of this invention is to provide a relatively inexpensive consumer load control means and method. A second object is to provide a means and method which will result in a very high degree of security against actuation of a receive circuit by a spurious signal. A still further object of this invention is to provide a system which is simple to use, wherein the constituent elements of both the transmitter and receiver are of rugged and simple construction and not likely to have maintenance problems, and yet wherein a large number of codes can be transmitted for a comparatively low cost to thereby control a large number of functions.